In wireless networks, comprising at least one base station and a plurality of subscriber stations, where the network is subject to the “near-far” problem and/or the like, the base station is preferably involved with the management of the transmission power levels of each subscriber station. In general, permitting a subscriber station to transmit with more power allows that subscriber station to achieve a higher data rate, but can also interfere with the transmissions of other subscriber stations. The tradeoffs between overall network efficiency versus performance of a particular subscriber station need to be carefully managed.
In addition to network limits, each subscriber station is limited in its own maximum uplink transmit power by regulatory and/or hardware limits. If a subscriber station transmits at an uplink transmit power level higher than its rated uplink transmit power, non-linear effects in the transmitter's power amplifier can produce errors in the channel. Additionally, for health and safety reasons and for regulatory reasons to minimize adjacent channel interference, wireless devices are restricted in the amount of power that they can transmit with, and this amount is typically less than the maximum possible output of the subscriber station's power amplifier. For example, a fixed wireless transmitter operating at 1.9 MHz may be restricted, by regulation, to a maximum uplink transmit power of 30 dBm. As known to those of skill in the art, specialized circuitry in the subscriber station's power amplifier, typically referred to as “foldback circuitry”, is often employed to limit outputted power and prevent the subscriber station from transmitting over-specification and/or outside of regulatory limits.
When the base station is responsible for admitting subscriber stations to a higher data rates, the base station needs to know how much available uplink transmit power (i.e., the difference between the maximum uplink transmit power and the current average uplink transmit power) is available for each subscriber station. The base station does not want to assign a data rate to the subscriber station that the subscriber station is unable to achieve due to an insufficient amount of available uplink transmit power. The base station can make an estimate of available uplink transmit power for a subscriber station by receiving reports of the current uplink transmit power uplink transmit power from that subscriber station. However, this estimate may not be accurate due to variations in the subscriber station's measurement due to operating temperatures in the circuitry, a lack of, or limited, calibration of circuitry in the subscriber station. Due to these inaccuracies, the base station may perceive that a subscriber station has more available uplink transmit power than is actually the case. As such, the base station may attempt to admit the subscriber station to a higher uplink data rate, requiring a higher uplink transmit power than the subscriber station can actually support, resulting in channel errors and a waste of network capacity.
To prevent this problem from occurring, it is typical to provide an uplink transmit power margin to the estimate of available uplink transmit power to account for variances in each subscriber station. However, providing too large an uplink transmit power margin results in a potentially lower maximum data rate for the subscriber station, while providing too small an uplink transmit power margin can result in communication link errors and a waste of network capacity. As such, it is desired to provide a system and method to determine more accurately the amount of available uplink transmit power available to a subscriber station in order to assign data rates to that subscriber station.